In the paper industry, it is often desirable to form multi-layered fibrous pulp webs, where different fiber suspensions are used in the different layers. A paper can for example consist of a core made of recycled fiber material while the surface layers consist of a different fiber material in order to give the paper a desired surface. Such multi-layered webs can be produced by means of layered head boxes, where the different fiber suspensions are separated by means of at least one blade. At the outlet of the head box, the fiber suspension layers are dispensed on top of each other on a paper forming wire and thereby forming a multi-layered fibrous pulp web. Since the different layers often consist of materials with different fiber properties or even different colors, it is essential to prevent mixing of the layers. The mixing is mainly caused by the wake that is formed behind the trailing edge of the blade. These wakes generate strong turbulent mixing, which effectively prevents the possibility to obtain a good layer purity. Several attempts to solve this problem have been made.
The British patent application GB, A, 2107751 discloses a paper machine and a process for producing a multi-layer fibrous pulp web, where two layers of fiber suspension are separated by an intermediate flow duct through which water is flowing and thereby forming a separating layer between the fiber suspension layers. Said flow duct extends from the inlet end of the head box up to approximately the center of the head box and consists of rigid partition walls. This construction requires a symmetrical position of the blades in the head box in order to achieve equal pressure on both sides of the blade at the trailing edge. Also, severe mixing will take place unless the partition walls forming the flow duct are extremely accurately positioned, which in reality is practically impossible. As a consequence of this, it is not possible to feed different volume flows of fiber suspension separated by the partition walls without causing an unwanted difference in pressure between the fiber suspension layers. The positioning of the partition walls in order to achieve equal pressure on both sides of the blade must furthermore be adapted for one specific speed of the fiber suspension, which makes the paper machine inflexible to use.